tetrodotoxin and Diabetic-Neuropathies

Diabetes mellitus (DM) is a common chronic medical problem worldwide; one of its complications is painful peripheral neuropathy, which can substantially erode quality of life and increase the cost of management. Despite its clinical importance, the pathogenesis of painful diabetic neuropathy (PDN) is complex and incompletely understood. Voltage-gated sodium channels (VGSCs) link many physiological processes to electrical activity by controlling action potentials in all types of excitable cells. Two isoforms of VGSCs, NaV1.3 and NaV1.7, which are encoded by the sodium voltage-gated channel alpha subunit 3 and 9 (Scn3A and Scn9A) genes, respectively, have been identified in both peripheral nociceptive neurons of dorsal root ganglion (DRG) and pancreatic islet cells. Recent advances in our understanding of tetrodotoxin-sensitive (TTX-S) sodium channels NaV1.3 and NaV1.7 lead to the rational doubt about the cause-effect relation between diabetes and painful neuropathy. In this review, we summarize the roles of NaV1.3 and NaV1.7 in islet cells and DRG neurons, discuss the link between DM and painful neuropathy, and present a model, which may provide a starting point for further studies aimed at identifying the mechanisms underlying diabetes and painful neuropathy.

Topics: Animals; Diabetes Mellitus; Diabetic Neuropathies; Humans; Islets of Langerhans; NAV1.3 Voltage-Gated Sodium Channel; NAV1.7 Voltage-Gated Sodium Channel; Sodium Channel Blockers; Tetrodotoxin

Type 2 diabetic mellitus (T2DM) has reached pandemic status and shows no signs of abatement. Diabetic neuropathic pain (DNP) is generally considered to be one of the most common complications of T2DM, which is also recognized as one of the most difficult types of pain to treat. As one kind of peripheral neuropathic pain, DNP manifests typical chronic neuralgia symptoms, including hyperalgesia, allodynia, autotomy, and so on. The injured dorsal root ganglion (DRG) is considered as the first stage of the sensory pathway impairment, whose neurons display increased frequency of action potential generation and increased spontaneous activities. These are mainly due to the changed properties of voltage-gated sodium channels (VGSCs) and the increased sodium currents, especially TTX-R sodium currents. Curcumin, one of the most important phytochemicals from turmeric, has been demonstrated to effectively prevent and/or ameliorate diabetic mellitus and its complications including DNP. The present study demonstrates that the TTX-R sodium currents of small-sized DRG neurons isolated from DNP rats are significantly increased. Such abnormality can be efficaciously ameliorated by curcumin.

Topics: Analgesics; Animals; Curcumin; Diabetes Mellitus, Type 2; Diabetic Neuropathies; Ganglia, Spinal; Insulin Resistance; Male; Neuralgia; Neurons; Pain Threshold; Rats, Sprague-Dawley; Sodium Channels; Tetrodotoxin

This study establishes a mechanism for metabolic hyperalgesia based on the glycolytic metabolite methylglyoxal. We found that concentrations of plasma methylglyoxal above 600 nM discriminate between diabetes-affected individuals with pain and those without pain. Methylglyoxal depolarizes sensory neurons and induces post-translational modifications of the voltage-gated sodium channel Na(v)1.8, which are associated with increased electrical excitability and facilitated firing of nociceptive neurons, whereas it promotes the slow inactivation of Na(v)1.7. In mice, treatment with methylglyoxal reduces nerve conduction velocity, facilitates neurosecretion of calcitonin gene-related peptide, increases cyclooxygenase-2 (COX-2) expression and evokes thermal and mechanical hyperalgesia. This hyperalgesia is reflected by increased blood flow in brain regions that are involved in pain processing. We also found similar changes in streptozotocin-induced and genetic mouse models of diabetes but not in Na(v)1.8 knockout (Scn10(-/-)) mice. Several strategies that include a methylglyoxal scavenger are effective in reducing methylglyoxal- and diabetes-induced hyperalgesia. This previously undescribed concept of metabolically driven hyperalgesia provides a new basis for the design of therapeutic interventions for painful diabetic neuropathy.

Topics: Animals; Cerebrovascular Circulation; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Humans; Hyperalgesia; Mice; Mice, Inbred C57BL; NAV1.8 Voltage-Gated Sodium Channel; Neural Conduction; Nociceptors; Pyruvaldehyde; Sodium Channels; Streptozocin; Tetrodotoxin

Experimental evidence has been presented to suggest that protein kinase Cbeta isoform-selective inhibitor LY333531 is effective at alleviating diabetic hyperalgesia. In the present study, we isolated small (< or =25 microm in soma diameter) dorsal root ganglion (DRG) neurons from control and streptozocin (STZ)-induced diabetic rats, and examined the acute action of LY333531 (1-1000 nM) on the tetrodotoxin-resistant Na(+) current (TTX-R I(Na)), which plays an essential role in transmitting nociceptive impulses, using the whole-cell patch-clamp method. TTX-R I(Na) in diabetic DRG neurons was enhanced in amplitude (71.5+/-3.6pA/pF, n=10 versus 41.2+/-3.3pA/pF, n=8) and was activated at more negative potentials (V(1/2), -15.1+/-1.3 mV versus -9.6+/-1.4 mV), compared with that in control neurons. Bath application of LY333531 acutely inhibited TTX-R I(Na) in both control and diabetic DRG neurons, and the degree of inhibition by the drug at concentrations of 1, 10 and 100 nM was significantly greater in diabetic DRG neurons than in control DRG neurons. Thus, TTX-R I(Na), which is upregulated in the diabetic state, is likely to be more potently inhibited by submicromolar concentrations of LY333531. These results suggest that an acute inhibition of TTX-R I(Na) by LY333531 attenuates the exaggerated excitability of DRG neurons in the diabetic state, which appears to be related at least partly to anti-hyperalgesic actions of the drug in diabetic neuropathy.

Topics: Animals; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Dose-Response Relationship, Drug; Enzyme Inhibitors; Ganglia, Spinal; Indoles; Male; Maleimides; Membrane Potentials; Neural Conduction; Neurons, Afferent; Patch-Clamp Techniques; Protein Kinase C; Protein Kinase C beta; Rats; Rats, Sprague-Dawley; Sodium Channel Blockers; Sodium Channels; Tetrodotoxin

Diabetic neuropathy is a common form of peripheral neuropathy, yet the mechanisms responsible for pain in this disease are poorly understood. Alterations in the expression and function of voltage-gated tetrodotoxin-resistant (TTX-R) sodium channels have been implicated in animal models of neuropathic pain, including models of diabetic neuropathy. We investigated the expression and function of TTX-sensitive (TTX-S) and TTX-R sodium channels in dorsal root ganglion (DRG) neurons and the responses to thermal hyperalgesia and mechanical allodynia in streptozotocin-treated rats between 4-8 weeks after onset of diabetes. Diabetic rats demonstrated a significant reduction in the threshold for escape from innocuous mechanical pressure (allodynia) and a reduction in the latency to withdrawal from a noxious thermal stimulus (hyperalgesia). Both TTX-S and TTX-R sodium currents increased significantly in small DRG neurons isolated from diabetic rats. The voltage-dependent activation and steady-state inactivation curves for these currents were shifted negatively. TTX-S currents induced by fast or slow voltage ramps increased markedly in neurons from diabetic rats. Immunoblots and immunofluorescence staining demonstrated significant increases in the expression of Na(v)1.3 (TTX-S) and Na(v) 1.7 (TTX-S) and decreases in the expression of Na(v) 1.6 (TTX-S) and Na(v)1.8 (TTX-R) in diabetic rats. The level of serine/threonine phosphorylation of Na(v) 1.6 and In Na(v)1.8 increased in response to diabetes. addition, increased tyrosine phosphorylation of Na(v)1.6 and Na(v)1.7 was observed in DRGs from diabetic rats. These results suggest that both TTX-S and TTX-R sodium channels play important roles and that differential phosphorylation of sodium channels involving both serine/threonine and tyrosine sites contributes to painful diabetic neuropathy.

Topics: Anesthetics, Local; Animals; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Ganglia, Spinal; Humans; Hyperalgesia; Intermediate Filament Proteins; Male; Membrane Glycoproteins; Membrane Potentials; Nerve Tissue Proteins; Neurons; Patch-Clamp Techniques; Peripherins; Phosphorylation; Protein Isoforms; Rats; Rats, Sprague-Dawley; Sodium; Sodium Channel Blockers; Sodium Channels; Tetrodotoxin

Streptozocin (STZ)-induced diabetic rats show hyperalgesia that is partially attributed to altered protein kinase C (PKC) activity. Both attenuated neuronal nitric oxide synthase (nNOS)-cGMP system and tetrodotoxin-resistant (TTX-R) Na channels in dorsal root ganglion neurons may be involved in diabetic hyperalgesia. We examined whether PKCbeta inhibition ameliorates diabetic hyperalgesia and, if so, whether the effect is obtained through action on neurons by testing nociceptive threshold in normal and STZ-induced diabetic rats treated with or without PKCbeta-selective inhibitor LY333531 (LY) and by assessing the implication of LY in either nNOS-cGMP system or TTX-R Na channels of isolated dorsal root ganglion neurons. The decreased nociceptive threshold in diabetic rats was improved either after 4 weeks of LY treatment or with a single intradermal injection into the footpads. The treatment of LY for 6 weeks significantly decreased p-PKCbeta and ameliorated a decrease in cGMP content in dorsal root ganglia of diabetic rats. The latter effect was confirmed in ex vivo condition. The treatment with NO donor for 4 weeks also normalized both diabetic hyperalgesia and decreased cGMP content in dorsal root ganglions. The expressions of nNOS and TTX-R Na channels were not changed with LY treatment. These results suggest that LY is effective for treating diabetic hyperalgesia through ameliorating the decrease in the nNOS-cGMP system.

Topics: Animals; Arginine; Cyclic GMP; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Enzyme Inhibitors; Ganglia, Spinal; Immunohistochemistry; Indoles; Male; Maleimides; NAV1.9 Voltage-Gated Sodium Channel; Neuropeptides; Nitric Oxide Synthase; Nitric Oxide Synthase Type I; Nociceptors; Protein Kinase C; Protein Kinase C beta; Rats; Rats, Sprague-Dawley; Sodium Channels; Tetrodotoxin

In the present study we tested the effects of the antihyperalgesic compound gabapentin on dorsal horn neurones in adult spinal cord. Slices were taken from control and hyperalgesic animals suffering from streptozocin-induced diabetic neuropathy. At concentrations up to 100 microM, bath application failed to affect the resting membrane properties of dorsal horn neurones taken from both groups of animal. In contrast, bath application of gabapentin dramatically reduced the magnitude of the excitatory postsynaptic current (EPSC) in neurones taken from hyperalgesic animals without altering the magnitude of the EPSC in control animals. Using a paired pulse stimulation protocol, together with analysis of miniature EPSC's, it was possible to demonstrate that gabapentin mediated these effects via a pre-synaptic site of action.

Topics: Acetates; Amines; Animals; Antimanic Agents; Cyclohexanecarboxylic Acids; Diabetic Neuropathies; Excitatory Postsynaptic Potentials; Gabapentin; gamma-Aminobutyric Acid; Hyperalgesia; Male; Membrane Potentials; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Spinal Cord; Streptozocin; Synaptic Transmission; Tetrodotoxin

The autonomic innervation of the seminal vesicle from 8 and 16 week streptozotocin-induced diabetic rats and age-matched controls was studied by pharmacological, histochemical and immunohistochemical methods. Contractions in response to electrical field stimulation, which were abolished using prazosin (2 microM) or tetrodotoxin (one to 1.6 microM), and to noradrenaline were significantly increased in both eight and 16 week diabetic animals. The contractile response to acetylcholine was significantly increased in the 16 week diabetic rats only, when compared with controls. Although these responses were significantly increased, no difference was found in ED50 and EF50 values between control and diabetic rats. Vasoactive intestinal polypeptide (0.3 microM) had no effect on resting tension or nerve-mediated responses. In seminal vesicles from control animals, both vasoactive intestinal polypeptide-immunoreactive and acetylcholinesterase-containing nerves were localised around the folds of the columnar epithelium of secretory cells, in contrast to neuropeptide Y-immunoreactive and catecholamine-containing nerves which were found in the smooth muscle layers. In seminal vesicles from both eight and 16 week diabetic animals no difference was seen in distribution or density of acetylcholinesterase-containing nerves; there was an increase in density and fluorescence intensity of vasoactive intestinal polypeptide- and neuropeptide Y-immunoreactive nerves and a decrease in catecholamine-containing nerves compared with controls. The results are discussed in relation to autonomic neuropathy in diabetes.

Topics: Acetylcholine; Adrenergic Fibers; Animals; Cholinergic Fibers; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Dose-Response Relationship, Drug; Electric Stimulation; Histocytochemistry; Immunohistochemistry; Male; Neuropeptide Y; Norepinephrine; Peptides; Prazosin; Rats; Seminal Vesicles; Tetrodotoxin; Time Factors; Vasoactive Intestinal Peptide